Light Emitting Diodes (LEDs) are typically produced by an epitaxial deposition of alloys of gallium nitride (GaN) on a transparent wafer substrate such as a sapphire. Many different types of defects may appear on either the substrate or the epitaxial layer. Some of these defects may be cracks in the sapphire which can then appear in the GaN layer, growth defects in GaN layer, pits, particles and stains from contaminates. In many cases, LED manufacturers will deposit GaN layers on patterned sapphire substrates (PSS). PSS serves to improve the light extraction efficiency by reducing the amount of light which is guided by total internal reflection. Manufacturers also desire to detect defects in the PSS since this will result in additional GaN growth defects or reduced light extraction efficiency. Similarly, defects in other transparent wafers will also result in degraded performance of the resulting device.
The inspection of transparent substrates is complicated by the difficulty of separating the scattered light reflected from the top and bottom surfaces. A diagram of a transparent substrate is shown in FIG. 1. Illustrated in FIG. 1 is a 1 mm thick glass wafer (sample 1). It is desired to inspect the top surface of the glass wafer for defects and to exclude the scattered light signal from the bottom surface of the glass wafer. In the prior art, Meeks (U.S. Pat. No. 7,907,269) describes a method to separate top and bottom surface scattered light. Meeks teaches the use of either an oblique or normal laser wherein bottom surface scattered light is separated by means of a beam block with a pinhole.